[PATCH v3 06/22] Add a function to kmap one page of a multipage bio_vec

[David Howells]

Add a function to kmap one page of a multipage bio_vec by offset (which is
added to the offset in the bio_vec internally).  The caller is responsible
for calculating how much of the page is then available.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: Paulo Alcantara (Red Hat) <pc@xxxxxxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: linux-block@xxxxxxxxxxxxxxx
cc: netfs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 include/linux/bvec.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index d36dd476feda..f834a862224e 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -299,4 +299,22 @@ static inline phys_addr_t bvec_phys(const struct bio_vec *bvec)
 	return page_to_phys(bvec->bv_page) + bvec->bv_offset;
 }
 
+/**
+ * bvec_kmap_partial - Map part of a bvec into the kernel virtual address space
+ * @bvec: bvec to map
+ * @offset: Offset into bvec
+ *
+ * Map the page containing the byte at @offset into the kernel virtual address
+ * space.  The caller is responsible for making sure this doesn't overrun.
+ *
+ * Call kunmap_local on the returned address to unmap.
+ */
+static inline void *bvec_kmap_partial(struct bio_vec *bvec, size_t offset)
+{
+	offset += bvec->bv_offset;
+
+	return kmap_local_page(bvec->bv_page + (offset >> PAGE_SHIFT)) +
+		(offset & ~PAGE_MASK);
+}
+
 #endif /* __LINUX_BVEC_H */